The present invention is a communication system for transmitting digital information from station to station. In particular, the present invention is directed to a communication system for use in the control and management of a power distribution system which includes a plurality of remote stations and a central station. Each of the remote stations transmits digital information such as the digital readings of watt-hour meters located in each remote station to the central station. Digital information is transmitted between each of the remote stations and the central stations according to the present invention by stopping and starting digital counters located in each of the stations in response to radio wave signals transmitted from station to station. A reference signal is supplied to the digital counters in each of the remote stations and the central station over the power lines connected between the stations.
In present power distribution systems it is frequently necessary to collect digital information such as watt-hour readings from a plurality of remote stations for purposes of load management and control. Under present practice, this digital information is manually collected by sending a person to read the watt-hour meters at each remote station. This digital information is then used for management purposes including billing of customers. However, because of the difficulty and expense involved in collecting this digital information, it is not collected very frequently. As a result, the potential uses for this digital information in load management and control are not fully realized.
Various communication techniques are known in the art for transmitting digital information from station to station. For example, techniques are known for transmitting digital information over the 60 cycle power lines in a power distribution system. However, because these power lines have been primarily designed to transmit power at a low frequency with as little power loss as possible, the injection of any information signals into these power lines requires the use of a communication technique that can utilize a relatively low frequency signal. One example demonstrating the use of a relatively low frequency communication signal in power lines is the ripple control communication technique wherein an information signal of a frequency in the order of the second to fourth harmonic of the power signal in the power lines of the power distribution network is injected into the power lines. This information signal can be detected by receivers located at appropriate points throughout the power distribution network. Although this ripple control communication technique is extremely accurate and there is such a high probability of acceptance of the information signal by the remote stations that redundancy or two-way checking is unnecessary, the low frequency requirement dictated by the use of the power lines of the power distribution system prevent the ripple control technique from being useful for communicating on an individual basis with more than a few hundred distinct stations.
The transmission capacity of the ripple control communication technique is primarily determined by the amount of time required to recognize an information signal at the receiving station. The selectivity of the receiving station as well as the frequency of the information signal affect the response time of the receiving station. Response time is determined by the amount of time required for a filter in the receiving station to reach a given amplitude sufficient to identify the presence of the information signal to the exclusion of transient interference. Additional time delay is encountered because this information signal must be present at this given amplitude for a time period at least as long as the response time of the filter. Because frequency selection dictates the band width of the receiver filter, a highly selective filter at 400 hz might have a band width of approximately 6 hz whereas a highly selective filter at 154 Mhz might have a band width of 6 Khz. The response time of a 6 hz filter approximates 150 milliseconds while the response time of a 6 Khz filter approximates 150 microseconds. Thus, the higher the frequency the shorter the response time of the filter in the receiving station and the higher the data rate capacity or transmission capacity of the communication system. For this reason, the use of the power lines of the power distribution system as the transmission medium dictates a rather low transmission capacity as compared with other possible communication techniques.
The communication capacity of the ripple control communication technique is also limited. In order to make maximum use of any transmission medium, it is desirable to use the least possible number of bits in coding the data signals to obtain the largest possible number of commands. The relationship between the number of bits and number of commands is determined by the formula C - 2.sup.n where C is the number of commands and n is the number of bits. Combining this formula with the previously determined response time (150 milliseconds) of a low frequency selective filter for the ripple control communication technique will determine the theoretical constraints of the ripple control technique. Thus, a thousand different commands would require 10 bits at 150 milliseconds per bit for a theoretical signal time of 1.5 seconds. However, in applying practical constraints, we would probably find the practical minimum signal time to be at least 5 seconds. If such a communication technique is used for individual interrogation of a plurality of remote stations, even on a one-way basis it would take a month for the transmission of information signals to a population of only 250,000 units. The communication capacity of the ripple control communication technique is clearly limited to mass addressing and supervisory control. In addition, the communication capacity of the ripple control technique cannot be improved by use of multiplexing because the ripple control technique necessarily saturates the power lines of the entire power distribution system.
It is possible to use other communication techniques other than the ripple control communication technique which use the power lines of a power distribution system as a transmission medium. For example, communication techniques such as frequency shift keying and pulse code modulation can be used. These communication techniques permit the use of multiplexing and other techniques for data compression. As a result, concurrent transmission among groups of points located in different sections of the power distribution system can be accomplished. However, these other communication techniques are also relatively slow when employed over 60 cycle power lines as compared with their use in other transmission media. In addition, if geographical multiplexing or partitioning of the power distribution system is used, a method must be devised for gaining access to the different points in the power distribution system.
Another possible known communication technique which may be utilized for transmission of digital information from a plurality of remotely located watt-hour meters to a central station is telephone transmission. Telephone transmission permits the use of tone modulation and other sophisticated digital transmission techniques. However, for purposes of load management and control in a power distribution system involving a large number of customers, the present high cost as well as the future potential for large cost increases prohibits the use of telephone transmission.
Analog radio transmission offers the greatest flexibility in frequency selection and, as a result, many different communication techniques can be combined with analog radio transmission. In addition, radio transmission is the most inexpensive of the available transmission media. However, radio transmission suffers a serious drawback in that it is not possible to establish a dedicated path between one station and another station to the exclusion of all other radio transmissions. Although the Federal Communications Commission allocates frequencies for different purposes and issues licenses for the use of these allocated frequencies, in practice there is often both intentional and unintentional violation of these frequency allocations. In order to sufficiently protect the integrity of an analog radio transmission system, the cost of the equipment involved would be beyond that justified for a communication system for load management and control in a power distribution system.
Another available communication technique is digital radio transmission. The integrity of digital radio transmission is much easier to safeguard than analog radio transmission because the receiving stations in a digital radio system can be simply preconditioned to accept certain sequences of digital pulses. As a result, the cost of digital radio transmission is much less than analog radio transmission. One way or outbound digital radio communication techniques have been proven acceptable for the transmission of digital information. For example, either a limited number of commands can be sent to a large grouping of similarly coded remote stations or a small number of remote stations can be individually addressed. However, using present digital radio transmission techniques, information cannot be brought back from a large number of remote stations if each of these remote stations must be distinctly identified. Thus, with respect to the communication of digital information between a plurality of remote stations and a central station in a power distribution system, the problem lies not with with the transmission capabilities or economics of digital radio transmission, but with the requirement for a communication technique which is capable of identifying individual stations and then allowing an individual station to communicate its digital information back to the central station.
A number of interconnected transmission systems are known which use a combination of the above mentioned communication techniques for the purposes of bi-directional communication between a small number of remote stations and a central station. For example, the ripple control technique, frequency shift keying or pulse code modulation may be used for transmitting digital information from a small number of remote stations to a substation which is connected by a telephone link with a central station. Although such a combination may use pulse code modulation to advantage for data acquisition and then use telephone transmission to advantage for speed in data transmission, the same disadvantage mentioned above with respect to each of these communication techniques still are present in this interconnected transmission system. Similarly, other known interconnected transmission systems do not eliminate the disadvantages of each of the communication techniques contained therein because such interconnected transmission systems are merely a series connection of several different communication techniques.
It is an object of the present invention to optimize the advantages of different communication techniques and to combine these different communication techniques in a new communication system. The communication system of the present invention is a comprehensive system which uses the optimum features of several different communication techniques without being subject to the individual disadvantages of these communication techniques.
It is an object of the present invention to develop a bi-directional communication system for transmitting a large amount of digital information between a central station and a large number of remote stations on an individual basis. The communication system of the present invention transmits this digital information at high speed while at the same time having the advantages of reliability, accuracy and low cost.
It is a further object of the present invention to develop a communication system which can be effectively used for load management and control in a power distribution system. In particular, it is an object of the communication system of the present invention to transmit digital information from a large number of remote stations in a power distribution system such as the watt-hour meters at customer locations to a central station for use in load management and control.
Another object of the present invention is to provide a digital communication system which protects data integrity by providing a high level of security. In particular, it is an object of the present invention to provide a communication system designed with a series of checks such that even minor interferences will prevent the completion of data transmission rather than allow incorrect data to be transmitted. Furthermore, the communication system of the present invention operates at high speed, a non-response followed by a reinterrogation is preferred rather than complicating the transmitted digital information with a combination of error detecting and error correcting codes.